In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, one parameter in providing good performance and capacity for a given communications protocol in a communications network is the ability to perform efficient network access. In general terms, in order for a terminal device to obtain successful network access to a cellular network, such as a Long Term Evolution (LTE) based communications network, the terminal device first needs to find a cell, perform a time and frequency synchronization and read system information. As an example, the initial network access procedure as performed by the terminal device in LTE can be summarized in the following five steps: (1) the terminal device detects the Primary Synchronization Signal (PSS) transmitted by the network node (denoted eNB in LTE), whereby the local oscillator in the terminal device is adjusted to the frequency of the network node, and timing on an orthogonal frequency-division multiplexing (OFDM) symbol level, (2) the terminal device detects the Secondary Synchronization Signal (SSS), providing the eNB Cell ID and frame timing, (3) the terminal device decodes a Physical Broadcast Channel (PBCH), providing Master Information Block (MIB) data comprising information such as cell downlink bandwidth, configuration on a Physical Hybrid-ARQ Indicator Channel (PHICH), where ARQ is short for automatic repeat request, which allows the Physical Downlink Control Channel (PDCCH) to be read, and System frame Number (SFN), (4) the terminal device decodes the PDCCH channel in order to determine which resource blocks (RBs) on a Physical Downlink Shared Channel (PDSCH) contains System Information Block (SIB) information, and (5) the terminal device decodes the PDSCH resource blocks containing the SIB information (such as cell operator, uplink bandwidth and power control, and random access information).
The above disclosed initial network access procedure enables the terminal device to receive information that is needed for the terminal device for establishing an operational connection to the network node. In order to establish an operational connection to the network node, the terminal device performs a random access procedure. As an example, the random access procedure as performed by the terminal device in LTE can be summarized in the following four steps: (1) the terminal device transmits a Random Access Preamble on a Physical Random Access Channel (PRACH) in a PRACH resource and using information in System Information Block 2 (SIB2), allowing the network node to estimate timing of the terminal device, thus enabling timing alignment, (2) the terminal device receives a Random Access Response (on the PDSCH channel) instructing the terminal device to modify its uplink timing, receive a Temporary Cell Radio Network Temporary Identifier (T-CRNTI), and be allocated uplink resources to be used for step 3, (3) the terminal device transmits its identity (on a Physical Uplink Shared Channel (PUSCH)), and (4) the terminal device receives a Contention Resolve Message (on the PDSCH channel), in case multiple terminal devices use the same PRACH resource.
In order to support high carrier frequencies, such as up to 100 GHz, a fifth generation (5G) communications network, called New Radio or NR by 3GPP, is under development. It is assumed that antenna beamforming will be used in such communications network. By using beamforming, the network node (denoted gNB in NR) is able to concentrate energy into certain directions, thereby having a further reach than what would otherwise be possible.
FIG. 1 is a schematic diagram illustrating a communications network 100. The communications network 100 comprises a network node 300 that is configured to provide network access to terminal devices 200a, 200b in a radio access network 110. The radio access network 110 is operatively connected to a core network 120. The core network 120 is in turn operatively connected to a service network 130, such as the Internet. The terminal device 200a is thereby, via the network node 300, enabled to access services of, and exchange data with, the service network 130. The network node 300 provides network access in the radio access network 110 by transmitting signals to, and receiving signals from, the terminal devices 200a, 200b in beams 140. The beams 140 will thus be used to transmit and receive data in different directions. The signals could be transmitted from, and received by, a transmission and reception point (TRP) 400 of the network node 300. The TRP 400 could form an integral part of the network node 300 or be physically separated from the network node 300.
Examples of network nodes 300 are radio access network nodes, radio base stations, base transceiver stations, Node Bs, evolved Node Bs, g Node Bs, access points, and access nodes. Examples of terminal devices 200a, 200b are wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, user equipment (UE), smartphones, laptop computers, tablet computers, network equipped sensors, network equipped vehicles, and so-called Internet of Things devices.
The beams 140 could be used to carry system synchronization signals and system information. As mentioned above, synchronization signals are divided into primary and secondary synchronization signals, PSS, SSS, respectively, and system information is transmitted in both PSS and SSS and in the PBCH channel. In some aspects, these signals together comprise a synchronization signal block (SSB). Further system information may be conveyed in the PDSCH channel.
FIG. 2 schematically illustrates an example of SSB transmission in a communications network 100 using antenna beamforming at the network node 300. As described above, the SSB is transmitted in beams 140a-140l in different directions from the network node 300. As in the example of FIG. 2 it could be that not all beams 140a-140l are swept within one synchronization burst set. Particularly, according to the illustrative example of FIG. 2 the network node 300, via the TRP 400, transmits SSBs 150a-150l in two synchronization signal bursts 150 within a respective synchronization burst set 170a, 170b. Beams 140a-140f are swept within a first synchronization burst set 170a and beams 140g-140l are swept within a second synchronization burst set 170b. Each SSBs 150a-150l is transmitted in a respective one of the beams 140a-140l. For simplicity, but without loss of generality, it is assumed that SSBs 150x is transmitted in beam 140x, where x=a, b, c, . . . 1.
Depending on the location of the terminal device 200a, 200b, a terminal device 200a, 200b could be able to find system information in the SSB of the beam 140a-140l that could serve the terminal device 200a, 200b. To each synchronization signal burst 150 there is (at least) one RACH occasion 160 in which a terminal device 200a, 200b receiving one of the SSBs, is assumed to transmit a random access preamble in order to establish an operational connection to the network node 300.
According to the illustrative example of FIG. 2, terminal device 200a receives SSB 150b in beam 140b and terminal device 200b receives SSB 150d in beam 140d. Although terminal devices 200a, 200b thus receive the system information in different SSBs they are still assumed to transmit a respective random access preamble within the same RACH occasion 160. However, since the same system information is provided in each SSB, the distance (t1 and t2, respective for terminal devices 200a, 200b in FIG. 2) to the RACH occasion 160 is not known.
One way to resolve this could be for the network node 300 to, in each synchronization burst set 170a, 170b, explicitly signal when the RACH occasion 160 occurs, but such signalling could create a large overhead and it could still be difficult for the terminal devices 200a, 200b to find the RACH occasion 160.
In view of the above, there is a need for an improved random access procedure in the network 100.